1. Field of the Invention
This invention relates broadly to Couette devices. More particularly, this invention relates to methods for using Couette devices to study emulsion fluid flow.
2. Description of the Related Art
A fluid is a substance that continually deforms or flows under an applied shear stress. It may contain liquids, gases, and solids, and generally takes on the shape of the container in which it is housed. When a fluid is transported through a pipe or tube, a number of properties of the fluid (such as temperature, pressure, and viscosity) may change depending on both the external constraints and the composition of the fluid. An emulsion is a fluid that consists of a mixture of at least two fluid phases that do not or only partially blend with each other. In a two-phase emulsion, one fluid (the dispersed phase) is dispersed within the other (the continuous phase). The creation of an emulsion from separate phases requires stirring, shaking, or some other form of energy input (microemulsions are not considered in this context). The process by which emulsions are created is called emulsification.
In an emulsion, the degree and uniformity of dispersion of the dispersed phase within the continuous phase will generally depend on the nature of the fluid phases of the emulsion, the rate of mixing, and the length of time that the fluid phases are mixed. If the interfacial tension between the dispersed and continuous phases of an emulsion is low or the kinetic stability of the thin liquid films between the approaching emulsion droplets is low, then the emulsion could be unstable. Over time, the components of an unstable emulsion tend to separate if the mixing, stirring, or shaking is ceased.
An emulsion's viscosity measurement represents its resistance to flow. This characteristic is frequently tested because it can directly affect the operation and performance of an industrial device. However, an emulsion's viscosity varies according to the specific proportions of its components and the extent to which they are mixed. Therefore, an emulsion's viscosity may vary under different flow conditions because the degree of emulsification (mixing) may vary in accordance with the flow conditions.
An emulsion's viscosity is measured with a rheometer or viscometer. One such device frequently used in the art is a Couette device. A cylindrical Couette device has an outer hollow cylinder and an inner cylinder that together define an annulus in the space between the exterior surface of the inner cylinder and the interior surface of the outer cylinder. The annulus is filled with an emulsion and the cylinders rotate relative to one another. This rotation imposes shear stresses on the emulsion resulting in flow. The properties of the fluid flow vary depending on the parameters imposed on the emulsion by the device (such as temperature, pressure, and rate of shear (angular velocity)), as well as the characteristics of the fluid itself (such as its viscosity and density). Examples of Couette devices are shown in U.S. Pat. No. 6,959,588 B2 and U.S. Pat. No. 5,959,194.
Both wide and narrow gap rheometers may be used to determine an emulsion's viscosity. The gap size in a cylindrical Couette device refers to the distance between the outer surface of the inner cylinder and the inner surface of the outer cylinder. A smaller (narrow) gap usually promotes laminar flow in the device by inducing high shear rates throughout the fluid. A wide gap provides lower shear stresses, and can operate in a turbulent fluid flow regime that more closely resembles emulsification conditions in industrial equipment, such as in a pipeline or centrifugal pump.
Industrial devices frequently transport emulsions containing mixtures of oil, water, or other substances. The performance of an industrial device is often linked to a number of properties of the emulsion, such as its viscosity. Emulsions are thus frequently created and tested to evaluate their properties when subjected to the kinds of shear stresses, temperatures, pressures, and other flow parameters imposed by industrial devices in the field. It is often difficult to measure an emulsion's viscosity immediately after a sample is taken from a field device. Under laboratory conditions, emulsions are frequently generated in one device and then tested in another device, and care must be taken so that the emulsion's properties will not change significantly as it is transferred from the generating device to the testing device.